I'm not in the US, but I've read that Nikki Haley, the South Carolina governor, would be the best choice for running mate, whoever gets the Republican nomination:
She's sane, even bright, has experience in governing a State, she's Indian American so she appeals outside Trump's white heartland; she managed to get the Confederate flag removed from the State Capitol following the horrific racist murders at an African American church in Charleston.
In fact, she's far more impressive than Hillary.
If Trump chokes to death mid-rant just after Inauguration, Haley would be a much better Republican President than they've had for decades. Not much competition though.
NY Times and others have suggested Gov Christie, but he's too much of a bruiser like Trump, so wouldn't widen the appeal to those who don't like bar- room brawlers.
Trump needs someone who could attract independents and retain the moderate Republicans he needs to have a chance of winning.
Oh, having said that, if Hillary were to be indicted during the election, or if Trump exhumes her skeletons and finds real dirt, then he could of course sail through to the White House.
I remember in the early 1970s when Nixon's Veep Spiro Agnew had to resign after being indicted for bribery. So, occasionally those claig calls "the elite" can get indicted. However, iirc, Agnew never served a day in jail.